This invention relates to septic tank systems, and more particularly pertains to passive devices for apportioning the flow of effluent from a settling tank into separate nitrification trenches or lines in a subsurface disposal field. The terms "leaching field" and "nitrification field" are considered synonymous.
The soil on a building site is sometimes used as a medium for treatment and disposal of wastewater, including sewage emanating from a residence. A septic tank system is a subsurface wastewater system consisting of a settling tank and a subsurface disposal field. By the use of a septic tank system, wastewater is absorbed into the soil. Installation of septic tank systems in an improper manner or has a detrimental effect on the public health and environment through contamination of land, groundwater, and surface waters.
In a typical septic tank system, wastewater is discharged from a residence or other building into a settling tank, widely known as a septic tank. The settling tank is a water-tight, covered receptacle typically including two compartments. The settling tank receives the discharge of wastewater from a building, separates settleable and floating solids from the liquid, digests organic matter by anaerobic bacterial action, stores digested solids through a period of detention, and allows clarified liquid, herein called effluent, to discharge for additional treatment and final disposal. The effluent ultimately is received by a plurality of nitrification trenches or lines in a nitrification field. Each nitrification trench usually includes a perforated pipe. Distribution devices apportion the flow of effluent to the several nitrification trenches.
The distribution devices are designed to avoid overloading of effluent within a single nitrification trench. Should overloading occur, effluent often rises to the ground surface, resulting in an unhealthy and malodorous condition. The system must then be pumped out to restore the system to working condition.
Although pumps or siphons are sometimes used in a septic tank system, gravity distribution is very widely used. The advantages of gravity distribution include low cost, ease of installation, and little maintenance. Gravity distribution type septic tank systems are subject, however, to poor distribution to the nitrification trenches and local overloading within trenches.
Parallel distribution gravity type septic tank systems are often used, particularly where the ground surface is of fairly level grade. In a typical parallel distribution type septic tank system, a conduit directs effluent from the settling tank to the inlet of a distribution box. The distribution box is also provided with several outlets. Each outlet is connected to a pipe which proceeds to a lateral nitrification trench. Therefore, it is necessary to provide each outlet on the same level in order to provide equal flow to the several outlet pipes. In practice, it is difficult to provide outlet holes which are on exactly the same level.
In a serial distribution type septic tank system, flow of effluent is first directed to a first nitrification trench and thereafter directed to other nitrification trenches. If a parallel type system is used with a sloping nitrification field, the system tends to overload first in the nitrification trench at the lowest grade. For this reason, serial distribution type septic tank systems are often used in sites where the ground surface is not of level grade. Effluent flow is first directed to the nitrification trench at the highest grade, and thereafter is directed to the nitrification trench at the next highest grade, and in the same manner is directed to the nitrification trenches in order of their grade until finally effluent is directed to the nitrification trench at the lowest grade.
Serial flow distribution as described in the immediately preceding paragraph is typically accomplished through the use of drop boxes. A drop box is constructed such that the inlet is at a first elevation and the outlet to the nitrification trench lateral is at a second elevation, lower than the first elevation; and the outlet to the next lower drop box is at a third elevation, which is at a level which is lower than the first elevation but higher than the second elevation. After the nitrification trench supplied by nitrification trench lateral (at the second elevation) is filled, effluent rises to the level of the third elevation to flow to the next lower drop box.
One description of drop box design is contained in North Carolina Administrative Code, title 18, section 1955 (1), which requires that the top of the trench outlet laterals (at the second elevation as hereinabove described) be two inches below the outlet to the next lower drop box (at the third elevation as hereinabove described). The conventional drop box design not only fully utilizes the nitrification trench supplied by the outlet at the second elevation, but in fact tends to overload the trench. Thus, the conventional drop box design often prematurely fails at the highest grade nitrification trench in an effort to avoid premature failure at the lowest grade nitrification trench. And, by completely filling the nitrification trench, the conventional drop box design causes the air at the top of the nitrification line to be forced out, inhibiting the on-site treatment process.
After being installed underground for a period of time, the drop box may become out of level, possibly impairing its function. As a drop box occupies a rather large volume of space, hydrostatic pressure and frost heave may force the drop box upward, in some cases actually to the ground surface. This problem is one reason why the drop box is often formed of concrete, which has a sizable weight density.